1. Field of the Invention
The invention relates to an amplifier arrangement comprising a first amplifier stage having a first input which is the inverting input of the amplifier arrangement, a second input which is the non-inverting input of the amplifier arrangement, and an output, a second ampliefier stage having an input which is coupled to the output of the first amplifier stage, at least a first output transistor and an output coupled to said transistor, a negative-feedback network comprising a first resistor arranged between the output of the second amplifier stage and the inverting input of the amplifier arrangement and a second resistor and a capacitor arranged in series between the inverting input and a reference potential, and a protection circuit for protecting the first output transistor, which protection circuit has an output.
Such an amplifier arrangement may be used in integrated power amplifiers for audio equipment. The output transistors of such amplifier arrangements must be operated in the SOAR range (Safe operating Area Rating) in order to preclude damage to the output transistors due to overloading by an excess current and/or an excess voltage.
2. Description of the Prior Art
Such an amplifier arrangement is described in the non-prepublished Netherlands patent application Nos. 8300078 (PHN 10.546) and 8302197 (PHN 10.712). The output of the protection circuit does not act on the base of the output transistors, as is customary, but on the input of the second amplifier stage. This results in a high loop gain of the protection loop, thereby avoiding distortion of the output signal when the protection circuit is not fully activated. In a frequency-compensated second amplifier stage the amplifier arrangement has an open-loop gain which rolls off as a function of the frequency with 6 dB/octave from the first angular frequency and with 12 dB/octave above a second angular frequency. In order to obtain a well-defined gain it is customary to apply negative feed-back to the amplifier by means of a negative-feedback network which is external to the integrated circuit and which is arranged between the output of the amplifier arrangement and the inverting input of the amplifier arrangement. Generally, this negative feedback network comprises a resistor arrangement between the output and the inverting input and a second resistor and a capacitor arranged in series between the inverting input and a reference potential. The capacitance of the capacitor is selected to obtain substantially full d.c. negative feedback. For the frequency spectrum of the signals to be amplified the reactance of the capacitor is so small that the negative feedback in this frequency spectrum depends almost wholly on the first and the second resistors. For a stable gain the negative feedback factor which is defined by these resistors should be smaller than a specific maximum value in order to ensure that the third angular frequency for which the openloop gain of the amplifier arrangement is equal to the inverse of the negative feedback factor, is equal to or preferably lower than the second angular frequency and is situated at some distance from this frequency. Within the frequency range of the amplifier arrangement the protection circuit described in said patent applications exhibits a portion which rolls off with the frequency and a frequency-independent portion around the maximum third angular frequency. The latter is necessary in order to obtain a stable loop gain the protection loop when the protection circuit is active. For the maximum third angular frequency, where the loop gain of the protection loop becomes unity, the phase-shift in the loop must be smaller than 180.degree. so that the frequency roll-off must be smaller than 12 dB/octave.
The frequency dependence of the protection circuit is achieved by a means of a frequency-dependent network comprising a resistor and a capacitor connected in series. When the load of the amplifier arrangement is short-circuited it is found that the gain-frequency characteristics substantially towards lower frequencies. In order to ensure that the amplifier arrangement with the protection circuit remains stable the frequency-independent portion of the gain of the protection circuit should cover a comparatively wide frequency range around the maximum third angular frequency. This is made possible by shifting the gain-frequency characteristic of the protection circuit towards the lower frequencies. However, this demands the use of larger capacitances which, when integrated, occupy a substantial integration area.